Objectives. The purpose of this study was to assess flow dynamics and flow capacities of internal mammary artery and saphenous vein grafts to the left anterior descending coronary artery. Background. The postoperative flow capacity of internal mammary artery grafts to the left anterior descending coronary artery has been reported to be restricted compared with that of saphenous vein grafts in studies using radionuclide angiography. A recently developed Doppler guide wire has been used to analyze the flow dynamics of bypass grafts and to clarify the mechanism of this limited flow capacity. Methods. Phasic flow velocity recordings were obtained in the midportion of the bypass graft and within the native left anterior descending artery, using a 0.018-in. (0.046-cm) 12-MHz Doppler guide wire, in 53 patients: 27 patients with an internal mammary artery graft (16 with a new graft assessed 1 month postoperatively and 11 with an old graft assessed at 1 year) and 26 patients with a saphenous vein graft (13 with a new graft assessed 1 month postoperatively and 13 with an old graft assessed at 1 year). All patients were studied at baseline rest and during hyperemia induced by intravenous infusion of dipyridamole, 0.56 mg/kg body weight, over 4 min. Results. In the left anterior descending artery itself, systolic and diastolic peak velocities, the time average of the instantaneous spectral peak velocity (time-averaged peak velocity), vessel diameter and the calculated flow volume did not differ significantly among the four graft groups. The time-averaged peak velocity was significantly greater for new than for old arterial grafts or for new or old vein grafts (mean ± SD 27 ± 9 vs. 19 ± 6, 11 ± 5 and 12 ± 6 cm/s, respectively, p Conclusions. Internal mammary artery graft flow early after operation is characterized by a higher rest velocity than that of vein graft flow. This high velocity maintains flow volume at baseline condition in compensation for the smaller diameter. Although flow reserve does not differ significantly between new and old vein grafts, that for internal mammary artery grafts is significantly reduced soon after bypass surgery. This restricted flow capacity improves late postoperatively because of an increase in diameter and a decrease in flow velocity from baseline levels.